Action of intercostal muscle contraction on ribs

In portion 1 of the rib cage where the muscles are close to the spinal joints and far from the cartilage, large moment caused dominant rib rigid rotation

during the intercostal muscles contraction. The respective effects are also consistent with those obtained with lung inflation experiment on human by De Troyer et al. [37]. However, regarding the intercostal muscles in portion 2 and 3, the moment for rib rotation is relatively small, therefore we could not ignore the deformations of the bones and cartilages happened in this region of the rib cage.

For the ease of explanation, the simulation results are shown in the middle of Fig. 2-28, and the mechanisms are shown in the left and right side of Fig.

2-28 with schematic diagram. Dotted lines illustrate the state after muscle contraction and the solid lines show the original shape of the rib cage. Red arrow shows the component of the muscle normal to the ribs.

The mechanism of external intercostal muscles locates in portion 2 was shown in Fig. 2-28-A. As mentioned in section 1.3.2, the mechanical action should be attributed as the summation action of the rib rotation and the deformation. For the rib rotations caused by the external intercostal muscles in portion 2, according to moment distribution in the Fig. 2-10-B, the moment could generate an upward rib rotation as the simulation result shows in Fig. 2-28-A. However, we could also observer some additionally caudal displacement in the lateral portion of the upper ribs (also as the displacement results illustrates in Fig. 2-17). That is the deformation part going to be explained in the left part of Fig. 2-28-A.

According to the anatomical structure of the upper ribs, because costochondral articulation is straight, the costal cartilage is normal to the sternum and the rib cage is symmetrical about the midsagittal plane, therefore the components of muscle forces going along the ribs could be balanced by the mirror symmetry components of muscle forces at the other

side of the rib cage. Furthermore, it is also because the normal muscle forces could mainly bend and deformed the ribs. Hence, in order to discuss the mechanism, the normal muscles forces were marked in the left part of Fig. 2-28-A with red arrows. When the muscle fiber contracts, we could observe the lateral portion of the upper rib is under a caudal muscle force and the sternal side is acted on a cranial muscle force. Therefore, during the upward rib rotation, the lateral portion of upper ribs was additionally bent caudally by the caudal muscle force. This deformation part also contributed to the cranial movement of the sternum during rib rigid rotation.

For the lower ribs, comparing with the upper ribs, the difference of the mechanism is caused by the oblique costal cartilages and the approximate right angle of the costochondral articulations. For the upper ribs, as explained above, the component runs along the ribs is balance through the sternum. However, for the lower ribs, because the costal cartilages goes obliquely, diverges under the sternum and the rib is approximately perpendicular to the costal cartilage. Therefore, the component runs along the ribs could not be balanced by the other side of the rib cage and just could be balanced by the reaction force from the abdominal issues as the blue arrow shows in 1-28 (because we want to investigate the rib motion caused by the intercostal muscles, therefore at this study we have not consider the abdominal wall, and the abdominal wall could be additionally considered after that [11]). Hence, the costochondral articulation could be spread and folded by the outward and inward muscle forces going along the ribs.

Detailed explanation for the lower ribs is shown in the right side of Fig.

2-28-A. The same as the upper ribs, the normal components of the muscles

caused the cranial displacement of the sternum and the caudal displacement of the lateral portion of the ribs. Regarding the component going along the ribs in the lower ribs, as the schematic diagram shows, this force running along the ribs could deform the costochondral articulation and rotate the costal cartilage in lateral-cranial direction. The costochondral articulation angle became large. The gradually increased cranial displacement in lateral portion of the ribs is because in this examination the separated fragments 8 gradually close to the costochondral articulation in caudal direction, therefore in caudal direction the moment gradually become small and the effect of deformation gradually became predominant, so that the lateral-cranial displacement of the ribs gradually reversed the caudal displacement to cranial. More importantly, although the moment for bucket-handle rotation in portion 2 is small, this deformation pattern of the costal cartilages could contribute to expand the chest in lateral direction as the respiratory effect of the bucket-handle rotation.

The mechanism during the internal intercostal muscle in portion 2 is shown in Fig. 2-28-B. According to the moment distribution of internal intercostal muscles in Fig. 2-10-B, the downward rib rotation could be generated as the simulation result shows in middle of Fig. 2-28-B. However, the cranial displacement was also obtained (Fig. 2-14 and Fig. 2-15) due to the deformation generated by the muscle forces. For the upper ribs (the left side of Fig. 2-28-B), the normal component to the rib of the muscles force generated the deformation that it pulled down the sternum and elevated the lateral portions of the ribs. For the lower ribs, as shown in the right side of Fig. 2-28-B, not only the normal components lowered the sternum and elevated the lateral portion of the ribs, but also the costal cartilage was

deformed by the component going along the rib in caudal direction and towards the midsagittal plane, so that the costochondral articulation angle became small, as the arrows of rib moving direction illustrates in Fig.

2-28-B. Furthermore, although the moment for expiratory rib rotation is small in portion 2, the midsagittal direction rotation of the costal cartilages and the bent ribs illustrate the reduce of the transverse diameter of the rib cage that is the deformation could contribute to the expiratory chest movement during internal intercostal muscle contraction in portion 2 of the rib cage.

Regarding the mechanical action of internal intercostal muscles in portion 3 (parasternal intercostal muscles), the mechanism is shown in Fig. 2-28-C.

According the moment distribution of internal intercostal muscles in Fig.

2-10-B, the parasternal intercostal muscles could cranially rotate the ribs and sternum. However, because the deformation was happened, therefore the sternum was slightly lowered as the simulation result shows in the middle of Fig. 2-28-B. This is because the sternum and the ribs were under the caudal and cranial muscle force, respectively, as shown in the left side of Fig. 2-28-C. In addition, for the lower ribs, the mechanism is shown in the right side of Fig. 2-28-C. Because the muscles force acts on the costal cartilage, therefore the component normal to the costal cartilage pulls the costal cartilage in lateral-cranial direction. Hence, this deformation could contribute to expand the chest in lateral direction as the respiratory effect of the bucket-handle rotation. This mechanism could help to explain the experimental results that why the sternum moved caudally during the parasternal intercostal muscles contraction [40].

Edge of costochondral articulations

Rib moving direction

A Location of

activated muscle group

Normal components of muscle force

B

C

Rib moving direction Muscle fiber

direction Simulation result

Simulation result

Simulation result

Upper ribs Lower ribs

Abdominal reaction

force

Fig. 2-28 Mechanical actions of the intercostal muscles in portion 2 and 3 (solid line: before muscle contraction; dotted line: after muscle contraction) and simulated deformation results (blue: before muscle contraction; orange:

after muscle contraction). A: Mechanical actions of upper (left side) and lower (right side) external intercostal muscles in portion 2. B: Mechanical actions of upper (left side) and lower (right side) internal interosseous intercostal muscles in portion 2. C: Mechanical actions of upper (left side) and lower (right side) parasternal intercostal muscles in portion 3.

2.3.4 The net effect during entire intercostal muscle